Review Article

J. Electron. Packag. 2017;139(3):030801-030801-11. doi:10.1115/1.4036239.

The packaging of electronic and microelectromechanical systems (MEMS) devices is an important part of the overall manufacturing process as it ensures mechanical robustness as well as required electrical/electromechanical functionalities. The packaging integration process involves the selection of packaging materials and technology, process design, fabrication, and testing. As the demand of functionalities of an electronic or MEMS device is increasing every passing year, chip size is getting larger and is occupying the majority of space within a package. This requires innovative packaging technologies so that integration can be done with less thermal/mechanical effect on the nearby components. Laser processing technologies for electronic and MEMS packaging have potential to obviate some of the difficulties associated with traditional packaging technologies and can become an attractive alternative for small-scale integration of components. As laser processing involves very fast localized and heating and cooling, the laser can be focused at micrometer scale to perform various packaging processes such as dicing, joining, and patterning at the microscale with minimal or no thermal effect on surrounding material or structure. As such, various laser processing technologies are currently being explored by researchers and also being utilized by electronic and MEMS packaging industries. This paper reviews the current and future trend of electronic and MEMS packaging and their manufacturing processes. Emphasis is given to the laser processing techniques that have the potential to revolutionize the future manufacturing of electronic and MEMS packages.

Topics: Lasers
Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2017;139(3):030802-030802-9. doi:10.1115/1.4037146.

Literature was reviewed and summarized on a few topics including: existing standards about the limits of devices' surface temperature, recent studies on the devices that caused discomfort and skin damage, human thermal sensation thresholds, the factors that affect thermal sensation, and the subjective ratings in the studies of thermal sensation. At the end, recent research on human subjective and objective testing was also summarized. The purpose of the review is to give an overview of cutaneous human thermal sensation and comfort, and how they are affected by the surface temperature of electronic devices.

Topics: Heat , Temperature , Skin , Testing
Commentary by Dr. Valentin Fuster

Research Papers

J. Electron. Packag. 2017;139(3):031001-031001-12. doi:10.1115/1.4036187.

Portable electronic devices are commonly exposed to shock and impact loading due to accidental drops. After external impact, internal collisions (termed “secondary impacts” in this study) between vibrating adjacent subassemblies of a product may occur if design guidelines fail to prevent such events. Secondary impacts can result in short acceleration pulses with much higher amplitudes and higher frequencies than those in conventional board-level drop tests. Thus, such pulses are likely to excite the high-frequency resonances of printed wiring boards (PWBs) (including through-thickness “breathing” modes) and also of miniature structures in assembled surface mount technology (SMT) components. Such resonant effects have a strong potential to damage the component, and therefore should be avoided. When the resonant frequency of a miniature structure (e.g., elements of an SMT microelectromechanical system (MEMS) component) in an SMT assembly is close to a natural frequency of the PWB, an amplified response is expected in the miniature structure. Components which are regarded as reliable under conventional qualification test methods may still pose a failure risk when secondary impact is considered. This paper is the second part of a two-part series exploring the effect of secondary impacts in a printed wiring assembly (PWA). The first paper is this series focused on the breathing mode of vibration generated in a PWB under secondary impact, and this paper focuses on analyzing the effect of such breathing modes on typical failure modes with different resonant frequencies in SMT applications. The results demonstrate distinctly different sensitivity of each failure mode to the impacts.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2017;139(3):031002-031002-10. doi:10.1115/1.4035850.

This paper explicitly establishes a modified creep model of a Sn–3.8Ag–0.7Cu alloy using a physical-based micromechanical modeling technique. Through experimentation and reformulation, steady-state creep behavior is analyzed with minimum strain rates for different temperatures 35 °C, 80 °C, and 125 °C. The new modified physical creep model is proposed, by understanding the respective precipitate strengthened deformation mechanism, seeing the dependency of the activation energy over the temperature along with stress and, finally, by integrating the subgrain-size dependency λss. The new model is found to accurately model the creep behavior of lead-free solder alloy by combining the physical state variables. The features of the creep model can be explored further by changing the physical variable such as subgrain size to establish a structure–property relationship for a better solder joint reliability performance.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2017;139(3):031003-031003-5. doi:10.1115/1.4036066.

The phosphor and die bonding configuration affect the optical efficiency and thermal performance in phosphor-coated white light emitting diodes (LEDs). In this paper, light emission studies reveal that the chromaticity shift and light extraction losses depend on the uniformity of phosphor particles deposited over the LED surface. A nonuniform and sparse phosphor layer affects the correlated color temperature (CCT) and the spectral Y–B ratio due to the disproportionate contribution of light emission between the LED device and the phosphor layer. Furthermore, the Y–B ratio was observed to reduce with temperature due to higher Stoke's energy and light extraction losses in the phosphor layer. As a result, the Y–B ratio exhibits an inverse relationship with the package's thermal resistance as a function of temperature. On the other hand, the thermal performance of a LED package is dependent on the die-bonding configurations (conventional and flip-chip). Due to the improved heat dissipation capabilities in flip-chip bonding, the temperature rise and thermal resistance of the package were observed to reduce with temperature. By alleviating the heat accumulation in the package, more stable colorimetric properties such as CCT and Y–B ratio can be achieved.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2017;139(3):031004-031004-8. doi:10.1115/1.4036368.

Fine pitch interconnects when used with two-dimensional (2D)/2.5D packaging technology offer enormous potential toward decreasing signal latency and by making it possible to package increased electrical functionality within a given area. However, fine pitch interconnects present their own set of challenges not seen in packages with coarse pitch interconnects. Increased level of stresses within the far back end of line (FBEOL) layers of the chip is the primary concern. Seven different types of 2D and 2.5D test vehicles with fine pitch and coarse pitch interconnects were built and tested for mechanical integrity by subjecting them to accelerated thermal cycling between −55 °C and 125 °C. Finite element based mechanical modeling was done to determine the stress level within the FBEOL layers of these test vehicles. For all the tested assemblies, experimental data and modeling results showed a strong correlation between reduced pitch and increased level of stresses and increased incidence of failures within the FBEOL region. These failures were observed exclusively at the passivation layer and aluminum pad interface. Experimental data in conjunction with mechanical modeling were used to determine a safe level of stress at the aluminum to passivation layer interface. Global and local design changes were explored to determine their effect on the stresses at this interface. Several guidelines have been provided to reduce these stresses for a 2D/2.5D package assembly with fine pitch interconnects. Finally, a reliable low stress configuration, which takes into account all the design changes, has been proposed, which is expected to be robust with very low risk of failure within the FBEOL region.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2017;139(3):031005-031005-11. doi:10.1115/1.4036817.

The performance of a novel impinging two-phase jet heat sink operating with single and multiple jets is presented and the influence of the following parameters is quantified: (i) thermal load applied on the heat sink and (ii) geometrical arrangement of the orifices (jets). The heat sink is part of a vapor compression cooling system equipped with an R-134a small-scale oil-free linear motor compressor. The evaporator and the expansion device are integrated into a single cooling unit. The expansion device can be a single orifice or an array of orifices responsible for the generation of two-phase jet(s) impinging on a surface where a concentrated heat load is applied. The analysis is based on the thermodynamic performance and steady-state heat transfer parameters associated with the impinging jet(s) for single and multiple orifice tests. The two-phase jet heat sink was capable of dissipating cooling loads of up to 160 W and 200 W from a 6.36 cm2 surface for single and multiple orifice configurations, respectively. For these cases, the temperature of the impingement surface was kept below 40 °C and the average heat transfer coefficient reached values between 14,000 and 16,000 W/(m2 K).

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2017;139(3):031006-031006-7. doi:10.1115/1.4036818.

In this paper, a series of bio-based epoxy resins containing organic silicone were prepared from eugenol through a mild synthetic route. Then, 4,4′-diaminophenyl methane (DDM) was applied to cure these epoxy resins, and bisphenol A epoxy resin (DGEBA) was used as a control. The chemical structures of the synthesized resins were characterized by nuclear magnetic resonance (1H-NMR). Properties of the cured epoxy resins were investigated by dielectric test, differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), dynamic mechanical analysis (DMA), and scanning electron microscopy (SEM). Compared with DGEBA, the bio-based epoxy resin containing cyclic organic silicon structure exhibited a dramatically lower dielectric constant at both low and high frequencies (3.46, 1 kHz, room temperature). Moreover, the silicone-modified bio-based epoxy resins demonstrated no weight loss below 325 °C and higher residues at 800 °C than that of DGEBA.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2017;139(3):031007-031007-6. doi:10.1115/1.4037145.

Encapsulation molding compounds (EMCs) are commonly used to protect integrated circuit (IC) chips. Their composition always contains fillers of a large amount (about 70%) and will affect the properties of the compounds. Thus, in order to clarify the filler effects, in this study, three types of silica fillers including crystal silica, edgeless silica, and fused silica were studied experimentally to explore their effects on the compounds. The results show that all of the flow spiral length, glass transition temperature (Tg), coefficient of thermal expansion (CTE), and water absorption rate of the encapsulation molding compounds decrease as the filler amount increases, irrespective of the filler type. In contrast, both thermal conductivity and flexural strength of the compounds increase as the filler amount increases, but also irrespective of the filler type. For the three fillers, the edgeless silica filler has the advantage of a large flow spiral length and can be molded better. It also has a larger thermal conductivity, larger flexural strength, and lower water absorption rate. Hence, for low stress industrial applications, the edgeless silica should be adopted as the filler of the encapsulation molding compounds.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2017;139(3):031008-031008-8. doi:10.1115/1.4037152.

The development of newer and more efficient cooling techniques to sustain the increasing power density of high-performance computing systems is becoming one of the major challenges in the development of microelectronics. In this framework, two-phase cooling is a promising solution for dissipating the greater amount of generated heat. In the present study, an experimental investigation of two-phase flow boiling in a micro-pin fin evaporator is performed. The micro-evaporator has a heated area of 1 cm2 containing 66 rows of cylindrical in-line micro-pin fins with diameter, height, and pitch of, respectively, 50 μm, 100 μm, and 91.7 μm. The working fluid is R1234ze(E) tested over a wide range of conditions: mass fluxes varying from 750 kg/m2 s to 1750 kg/m2 s and heat fluxes ranging from 20 W/cm2 to 44 W/cm2. The effects of saturation temperature on the heat transfer are investigated by testing three different outlet saturation temperatures: 25 °C, 30 °C, and 35 °C. In order to assess the thermal–hydraulic performance of the current heat sink, the total pressure drops are directly measured, while local values of heat transfer coefficient are evaluated by coupling high-speed flow visualization with infrared temperature measurements. According to the experimental results, the mass flux has the most significant impact on the heat transfer coefficient while heat flux is a less influential parameter. The vapor quality varies in a range between 0 and 0.45. The heat transfer coefficient in the subcooled region reaches a maximum value of about 12 kW/m2 K, whilst in two-phase flow it goes up to 30 kW/m2 K.

Commentary by Dr. Valentin Fuster
J. Electron. Packag. 2017;139(3):031009-031009-10. doi:10.1115/1.4037144.

Over the past ten years, there has been an exponential growth in innovations and designs to offer cutting edge electronic devices that are smaller, faster, with advanced features built in. The existence of smartphones, wearable devices, tablets, etc., is the evidence of these advancements. Original equipment manufacturers are looking for processes in electronic assemblies that can offer high‐speed, high accuracy, and fine droplet mass deposition to address the challenges of product miniaturization, high-density component packaging, and complex designs. This work presents a novel piezoelectric-driven jetting system that is designed to dispense small droplet masses with high accuracy and speed. The system is referred to as “novel” because, in the contact style jetting arena, the piezoelectric drive assembly for use to drive the motion of the piston assembly at high frequencies (up to 500 Hz) is new; furthermore, the piston motion tracking feature is patent pending of. Using a split-plot design of experiments (DOE) model, the jetting system is studied to understand the influence of critical designs introduced and to further review the smallest droplet mass possible to reliably dispense. This experimental analysis uses some of the widely used adhesives in the electronics assembly applications.

Commentary by Dr. Valentin Fuster

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